The present invention relates to a method of and an apparatus for melting glass having high reducing activity when melted, such as, for example, phosphate glass.
Generally, a glass of high optical quality having no striae, bubbles and so on is produced in the following manner. That is, a melting vessel formed of platinum or alloy thereof is accommodated in a protective case formed of refractory material. The melting vessel and the protective case are arranged within a melting furnace. Glass raw material is charged into the melting vessel, and is heated by an SiC resistance heating element or the like, to melt the glass raw material. The molten glass is further defoamed and homogenized.
Phosphate glass having reducing activity particularly strong when melted is highly reactive, and tends to react with the platinum or alloy thereof of the melting vessel to form a platinum-phosphorus alloy. A melting point of the platinum-phosphorus alloy is 588.degree. C. which is extremely low. Accordingly, if the melting vessel and a stirring rod, which are formed of platinum or alloy thereof, are employed to melt the phosphate glass, the inner surface of the wall of the melting vessel and an outer peripheral surface of the stirring rod in contact with the molten glass turn into a platinum-phosphorus alloy which has the low melting point. Thus, the melting vessel and the stirring rod are broken off locally for a short period of time. Additionally or alternatively, cracks are developed locally in the melting vessel and the stirring rod for a short period of time.
For the reason discussed above, the melting vessel and the stirring rod formed of platinum or alloy thereof are not used in melting the phosphate glass with strong reducing activity. Employed to melt such glass are a pot or a crucible and a stirring rod formed of ceramic such as clay, quartz glass or the like, or a melting vessel or a stirring rod formed or iridium or alloy thereof which is disclosed in West German Patent DE No. 1,906,717.
When the ceramic melting vessel such as clay crucible, quartz glass crucible or the like and the ceramic stirring rod are used to melt the phosphate glass having particularly strong reducing activity when melted, there arises the following problem. That is, the crucible and the stirring rod are eroded violently under strong erosion activity of the molten phosphate glass, so that the components of the melting vessel and the stirring rod are dissolved into the molten glass, resulting in striae, bubbles and so on. Thus, the optical quality of the molten glass is deteriorated considerably.
Further, if the phosphate glass having strong reducing activity is melted by the use of the melting vessel and the stirring rod formed of iridium or alloy thereof, it is possible to obtain a glass of high optical quality. However, there are the following problems. That is, the iridium and alloy thereof are extremely expensive as compared with the platinum and alloy thereof. In addition, the iridium or alloy thereof is a hard material and is low in workability so that it is difficult to obtain each of the melting vessel and the stirring rod with a large and complicated configuration.
Furthermore, there is also the following problem. That is, the iridium or alloy thereof tends to be oxidized so that the outer surface of the melting vessel is required to be coated with metal such as rhodium or the like. Thus, it takes a long time and much labor in manufacture of the melting vessel.